A number of situations exist where it is desired to provide communication between a computer controlled central station and a plurality of substations. If speed is required, the communication is usually provided by a parallel bus in which each bit of a frame in a message is conveyed from the central station to a substation by a separate conductor, but if there is sufficient time, the expense of isolation means can be avoided by using a serial bus in which the bits of each frame are conveyed in sequence via a single conductor.
A number of serial bus systems are known e.g., the RS232C industry standard and modifications thereof. They often require constant polling of the substations so that asynchronous operation in which any substation can request servicing by the central station is not available. Furthermore, in some systems the substations are not able to acknowledge the receipt of data or to give an indication to the central station of its accuracy.